As an interesting note, there was recent news of a 16 year old girl who hasn't aged at all and still looks and acts like a toddler. Her name is Brooke Greenberg and has baffled doctors. Even though shes only 16, no sign of aging is essentially immortality.

Just a thought but I assumed there are various ways to kill a vampire, so it would seem that no immortal vampire has ever existed. Not to say there hasn't been one or more but removing the head from the rest of the body and burning all of it, seems to keep it from continuning his/her blood sucking antics.